如何使用反射访问对象中的字段值 [英] How to access a field's value in an object using reflection

问题描述

我的问题：如何克服 IllegalAccessException 使用反射访问对象字段的值。

My Question: How to overcome an IllegalAccessException to access the value of an object's field using reflection.

扩展：我正在尝试学习反思，以使我的一些项目更通用。我在尝试调用 field.getValue（object）时遇到 IllegalAccessException 以获取该字段的值在那个对象。我可以得到名字和类型。

Expansion: I'm trying to learn about reflection to make some of my projects more generic. I'm running into an IllegalAccessException when trying to call field.getValue(object) to get the value of that field in that object. I can get the name and type just fine.

如果我将声明从 private 更改为 public 然后这很好用。但是为了遵循封装的规则，我不想这样做。任何帮助将不胜感激！谢谢！

If I change the declaration from private to public then this works fine. But in an effort to follow the "rules" of encapsulation I don't want to do this. Any help would be greatly appreciated! Thanks!

我的代码：

package main;

import java.lang.reflect.Field;

public class Tester {

  public static void main(String args[]) throws Exception {
    new Tester().reflectionTest();
  }

  public void reflectionTest() throws Exception {
    Person person = new Person("John Doe", "555-123-4567", "Rover");
    Field[] fields = person.getClass().getDeclaredFields();
    for (Field field : fields) {
      System.out.println("Field Name: " + field.getName());
      System.out.println("Field Type: " + field.getType());
      System.out.println("Field Value: " + field.get(person));
      //The line above throws: Exception in thread "main" java.lang.IllegalAccessException: Class main.Tester can not access a member of class main.Tester$Person with modifiers "private final"
    }
  }

  public class Person {

    private final String name;
    private final String phoneNumber;
    private final String dogsName;

    public Person(String name, String phoneNumber, String dogsName) {
      this.name = name;
      this.phoneNumber = phoneNumber;
      this.dogsName = dogsName;
    }
  }
}

输出：

run:
Field Name: name
Field Type: class java.lang.String
Exception in thread "main" java.lang.IllegalAccessException: Class main.Tester can not access a member of class main.Tester$Person with modifiers "private final"
    at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:95)
    at java.lang.reflect.AccessibleObject.slowCheckMemberAccess(AccessibleObject.java:261)
    at java.lang.reflect.AccessibleObject.checkAccess(AccessibleObject.java:253)
    at java.lang.reflect.Field.doSecurityCheck(Field.java:983)
    at java.lang.reflect.Field.getFieldAccessor(Field.java:927)
    at java.lang.reflect.Field.get(Field.java:372)
    at main.Tester.reflectionTest(Tester.java:17)
    at main.Tester.main(Tester.java:8)
Java Result: 1
BUILD SUCCESSFUL (total time: 0 seconds)

推荐答案

在您获取私有字段之前，您需要调用 setAccessible（true）; 在相应字段上：

Before you get a private field, you need to call setAccessible(true); on the corresponding field:

for (Field field : fields) {
  field.setAccessible(true); //Additional line
  System.out.println("Field Name: " + field.getName());
  System.out.println("Field Type: " + field.getType());
  System.out.println("Field Value: " + field.get(person));
}

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